Special handling of packages main page

[Julow]

Every pages and modules in a package must have a parent page, which is sometimes generated by the driver.
Drivers also want a "package list" page, listing all the packages they have build documentation for.

This is hard to implement with the current --parent flag, as the "package list" page becomes a dependency of everything else.
Drivers currently have to use a dummy page (for example, empty) as the parent of every packages and to swap this dummy page for the final "package list" page in the html output.

How to remove the need to pass the --parent when building the package main page while supporting the "package list" page and the other things we'll want in the future (cross package references, references to a package main page, package-scroped navigation side bar) ?